The present invention, in some embodiments thereof, relates to energy conversion and, more particularly, but not exclusively, to a direct liquid fuel cell system, which utilizes ammonia borane or derivatives thereof as fuel, and to applications employing a fuel cell system.
A fuel cell (FC) is an electrochemical device that continuously converts chemical energy directly to electrical energy as long as a fuel (commonly hydrogen, or hydrogen-containing compounds) and an oxidant (commonly oxygen) are supplied. One of the main advantages of fuel cells is their high energy density (typically 8,000-9,000 Wh/kg), which is about 18 times higher than conventional electrochemical power sources (such as, for example, Pb—PbO2; Zn—O2; Zn—Ag; Ni—Cd; Li-ion etc.).
Fuel cells are characterized by high efficiency compared to internal combustion engines. In addition, fuel cells are ecologically friendly and several types can function at temperatures as high as 100° C.
The development of fuel cells is one of the main directions in the field of new power engineering. Several types of fuel cells based on H2/O2, phosphoric acid, molten carbonate, alkaline, proton exchange membrane, direct methanol and solid oxide were developed in the last two decades [Carrette et al., Chem Phys Chem. 2000, 1, 162; Springer et al., J. Electrochem Soc. 1991, 8, 2334; Atkinson et al., Nature, 2004, 3, 17; Steele and Heinzel, Nature, 2001, 14, 345]. However, these fuel cells are still far from mass production due to multiple practical limitations.
Some of the obstacles associated with fuel cell development include complex electrode and cell design, catalysts poisoning and mechanical instability, high catalyst cost, low potential and slow oxidation kinetic.
In the last years, research efforts were focused on hydrogen-rich boron compound derivative-based fuel cells such as sodium borohydride fuel cells [see, for example, Miley et al. J. Pow Sour. 2007, 165, 509; Amendola et al. J. Pow Sour. 1999, 84, 130; Leon et al. J. Pow Sour. 2006, 155, 172; U.S. Pat. Nos. 6,562,497, 6,758,871 and 6,630,226; Leon et al. J. Pow Sour. 2007, 164, 441; Raman and Shukla, Fuel cell, 2007, 3, 225; Wee, J. H. J. Pow. Sour. 2006, 155, 329; Li et al. J. Electrochem. Soc. 2003, 150, A868; Liu et al. J. Pow. Sour. 2008, 175, 226; Kim et al. J. Electrochem. Soc. 2004, 151, A1039; and Choudhury et al. J. Pow. Sour. 2005, 143, 1].
Sodium borohydride is stable in solid state, and is characterized by an electrical capacity of 5,670 Ah/kg, energy density of 9,300 Wh/kg and hydrogen content of 11% (w/w). Sodium borohydride has no kinetic limitations, especially when utilized in the presence of noble metal catalysts. The standard potential of reduction (E0) of sodium borohydride (BH4−) is −1.24 V (see, equation 1 below).BH−4+8OH−→BO−2+6H2O+8e− E°=−1.24 V  (1)
One of the first NaBH4-based fuel cells, developed by Amendola et al. (supra), consists of BH4− solution as fuel (in 6M NaOH), O2 as oxidant, OH− ion as conducting membrane and Au/Pt anode and cathode as catalysts. This fuel cell functions only at 70° C., and shows high specific power density of 60 mW/cm2 (I=120 mA/cm2). This type of fuel cells, however, suffers from several drawbacks: the use of noble metal catalysts for increasing the cell efficiency is both costly and impractical, because BH4− is not chemically stable in the presence of such catalysts (due to hydrogen gas evolution), especially at the cell's working temperature (70° C.); the cathode/anode poisoning as a result from the existence of CO in air (carbonization), which requires a special scrubbing device to remove CO from air inlet; the inherent instability of BH4− anion in alkali solutions other than concentrated alkali solutions (>6M NaOH), with the latter being user unfriendly; and the slow kinetic of oxygen reduction.
Sodium borohydride-based fuel cells which utilize hydrogen peroxide as oxidant have therefore been developed (see, for example, Walsh et al, supra). These fuel cells have all the above-described inherent disadvantages of BH4−/noble metal catalysts, and, moreover, a concentrated alkali solution (6M NaOH) in the anode compartment and a concentrated acidic solution (2M HCl) in the cathode compartment are used.
NaBH4\H2O2 fuel cells which use metal catalysts (for anode and cathode) and Nafion-961 membrane, have also been developed (see, Shukla et al, supra). These fuel cells are characterized by modest current density and are further disadvantageous for using concentrated acidic and basic solutions.
Ammonia-borane (AB, NH3BH3) has recently been suggested as an alternative hydrogen-rich boron material. Ammonia borane (or borazane) is characterized by an electrical capacity of 5200 Ah/kg, energy density of 8400 Wh/kg (as NaBH4) and hydrogen content of 19% (w/w). AB is stable in aqueous solutions at pH≧6.5, in contrast to BH4−. The standard potential of reduction (E0) of Ammonia-borane is −1.216 V (see, equation 2 below).NH3BH3+6OH−→BO2−+NH4++4H2O+6e− E0=−1.216 V  (2)
Yao et al. [Journal of Power Sources 2007, 165, 125; referred to herein throughout as Zhung] described a fuel cell consisting of 0.5M AB (2M NaOH)—Ag catalyst//air/MnO2 catalyst. The cell produces an open circuit potential (EOCP) of −1.15 V, a current of 1 mA/cm2 for EW=0.9 V, a current of 2 mA/cm2 for EW=0.8 V and a current of 10 mA/cm2 for EW=0.4 V.
Zhang et al. [J. Pow. Sour. 2007, 168, 167; referred to herein throughout as Xu-1] describe a fuel cell consisting of AB (2M NaOH)-air fuel cell using Pt catalyst (0.15 mg/cm2 for anode and cathode). In this fuel cell, thiourea (1 mM) was added to the background electrolyte in order to prevent fuel spontaneous hydrolysis (decomposition). The cell produces a current of 24 mA/cm2 (EW=0.8 V) at RT. Zhang et al. [J. Pow. Sour. 2008, 182, 515; referred to hereinthroughout as Xu-2] further described fuel cell that consists of anode −0.5M AB (2M NaOH)/Pt-0.9 mg/cm2//cathode Pt-1.3 mg/cm2, humidified O2. Pump was used for fuel supply and fan was used for air (O2) supply. The cell produces a current of 50 mA/cm2 at EW=0.75 V (EOCP=−1.08 V).
U.S. Patent Application having Publication No. 2007/0151153, by Xu et al. describes a method of hydrogen generation which is effected by contacting ammonia borane with a metal catalyst, a solid acid or carbon dioxide, and further teaches using the generated hydrogen as fuel for fuel cells. JP Patent Application No. 2006-286549 teaches a direct liquid type fuel cell that utilizes an aqueous solution of a borane ammonium compound.
Additional background art includes U.S. Pat. No. 7,544,837 and U.S. Patent Application having Publication No. 2007/0128475, which teach a method of dehydrogenating an amine-borane using a base metal catalyst. The method, according to the teachings of these documents, may be used to generate H2 for portable power sources, such as fuel cells; and U.S. Pat. No. 7,285,142, which teaches a hydrolytic in-situ hydrogen generator that contains an amine borane (AB) complex in a solid or a slush form, at least one hydrogen generation catalyst, being an inorganic metal complex of the platinum group of metals, and water or other hydroxyl group containing solvent.
Further background art includes a review by Demirchi and Miele [Energy & Environmental Sci, 2009, DOI 10. 1039/b900595a)], in which sodium borohydride-based fuel cells vs. ammonia borane-based fuel cells are discussed.
Hydrogen peroxide (H2O2) is characterized by high standard potential of reduction (E0) of 1.77 V, and is therefore considered as a potent oxidant.
An AB/H2O2 fuel cell is characterized by a theoretical energy of 15,500 W/kg while a SB/O2 fuel cell is characterized by a theoretical energy of only 9,400 W/kg [see, for example, Demirchi's review, supra].
In the last years, non noble cathodes such as MnO2 were used as catalysts for the electro-oxidation of hydrogen peroxide, but were found ineffective [see, for example, as review by Walsh at al., J. power sources, 155 (2006) 172].
Lead sulfate (PbSO4) was also used as hydrogen peroxide (H2O2) catalyst [A. Shukla, Fuel cell 07; (2007) No. 3; 225-231].
Many electrochemical H2O2 sensors were fabricated, based on different electron mediators such as Prussian blue [Arkady et al. Anal. Chem., 1995, 67 (14), pp 2419-2423], ferrocene (FeC) [Mulchandani et al., Anal. Chem. 1995, 67, 94-100] and others [see, for example, A. Shinishiro; Chem. Sens, v.21 sup.B (2005) 61], however, the methodologies utilizing such catalysts produced a relatively low current.
Shukla et al. described the use of Prussian blue (PB) as an inorganic electron-transfer mediator (on carbon black; C/PB and polymer) as a catalyst for H2O2 reduction in a SB/H2O2 fuel cell [Shukla at al., J. Power sources, 2008, 178, 86]. The taught C/PB electrode was associated with a complicated fabrication protocol and a modest current density of about 35 mA/cm2.
Ferrocene is known as a potent electron-transfer mediator [see, for example, Anthony et al., Anal. Chem., 1984, 56, 667-671; Gagne et al., Inorg. Chem. 1980, 19, 2854-2855]. Ferrocene is chemically stable in acid solutions and is characterized by good absorption to carbon materials (via π-π interaction).
Attempts to adapt C/Fc for fuel cell technology have been described [see, for example, U.S. Pat. No. 7,320,842; and K. Gong, Science, 2009, 223, 760]. The described methodologies, however, involved a treatment at a temperature of 700° C., which results in decomposition of the C/Fc catalyst.
Additional art includes Logan B. E. and Regan J. M., Environmental Science & Technology, Sep. 1, 2006, 5172-5180.